Abstract
The Moderate Resolution Imaging Spectroradiometer vegetation continuous fields (MODIS VCF) Earth observation product is widely used to estimate forest cover changes, parameterise vegetation and Earth System models, and as a reference for validation or calibration where field data is limited. However, although limited independent validations of MODIS VCF have shown that MODIS VCF's accuracy decreases when estimating tree cover in sparsely-vegetated areas, such as in tropical savannas, no study has yet assessed the impact this may have on the VCF based tree cover distributions used by many in their research. Using tropical forest and savanna inventory data collected by the TROpical Biomes In Transition (TROBIT) project, we produce a series of corrections that take into account (i) the spatial disparity between the in-situ plot size and the MODIS VCF pixel, and (ii) the trees' spatial distribution within in-situ plots. We then applied our corrections to areas identified as forest or savanna in the International Geosphere-Biosphere Programme (IGBP) land cover mapping product. All IGBP classes identified as savanna show substantial increases in cover after correction, indicating that the most recent version of MODIS VCF consistently underestimates woody cover in tropical savannas. We estimate that MODIS VCF could be underestimating tropical tree cover by between 9–15 %. Models that use VCF as their benchmark could be underestimating the carbon uptake in forest-savanna areas and misrepresenting forest-savanna dynamics. While more detailed in-situ field data is necessary to produce more accurate and reliable corrections, we recommend caution when using MODIS VCF in tropical savannas.
Highlights
Tree cover values derived from Earth observation (E.O.) data form a fundamental part of ecological research
The in-situ field data was sourced from the ‘TROpical Biomes InTransition’ project (TROBIT) which include the corner locations and the Canopy Area Index (CAI) values for 24 forest and 24 savanna sites distributed across Australia, Brazil, Bolivia, Cameroon, and Ghana (Fig. A1 and Table A1, Fig. 2 in Torello-Raventos et al, 2013)
While MODIS VCF is a powerful and accessible tool to map tree cover, our field data-based corrections 290 indicate that the latest MODIS VCF collection 6 is missing a lot of woody cover even when uncertainty introduced by site canopy overlap and clumping within the MODIS VCF pixel are accounted for
Summary
Tree cover values derived from Earth observation (E.O.) data form a fundamental part of ecological research. They are used to estimate forest cover change, biomass, and carbon stocks (Bastin et al, 2019; Giriraj et al, 50 2017; Saatchi et al, 2011; Song et al, 2014); help identify key areas for conservation efforts Its quantitative measure of woody cover is recorded annually and is described as a percentage of ground cover, making it suited for use in evaluating dynamic global models (Lasslop et al, 2018; Rabin et al, 2017), as a proxy for in-situ data that is harder to collect (Kelley et al, 2019), and to help define parameters for 65 calculating global tree restoration potential (Bastin et al, 2019).
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